1. Technical Field
The present invention relates to a wiring board on which a semiconductor element is mounted.
2. Background
Recently, electronic devices including a portable game machine and a communication device are further reduced in size and increased in speed, so that a wiring board used in those devices is required to transmit a signal to densely-formed wiring at high speed. In order to realize the high-speed transmission, there is a system called a differential transmission system using a high-frequency signal.
As disclosed in Unexamined Japanese Patent Publication No. 2007-201221, in the differential transmission system, a signal is transmitted through differential lines formed of two parallel strip-shaped conductors. A voltage signal having a different polarity is transmitted from a transmission unit to each of the strip-shaped conductors, and a difference between the signals is taken and read in a reception unit, so that the signal can be easily read without the need to increase an amplitude of the signal to be transmitted to each of the strip-shaped conductors. Therefore, a time for forming the amplitude of the signal can be reduced, so that the signal can be transmitted at high speed.
FIG. 4 is a schematic cross-sectional view of a conventional wiring board B using the differential transmission system. The wiring board B includes a core insulating plate 11, an insulating layer 12, a wiring conductor 13, and a solder resist layer 14. A mounting portion 11a on which a semiconductor element is mounted is formed in a center of an upper surface of the wiring board B. The insulating plate 11 has a plurality of through-holes 15 penetrating from its upper surface to lower surface. The two insulating layers 12 are laminated on each surface of the insulating plate 11, and a plurality of via-holes 16 are formed in the insulating layer 12. The wiring conductors 13 are deposited on the surface of the insulating plate 11 and in the through holes 15, and on the surface of the insulating layer 12 and in the via-holes 16. The wiring conductor 13 includes differential lines 13a formed of two parallel strip-shaped conductors. Grounding or power supply conductors 13b are formed on the surfaces of the insulating plate 11 and the insulating layer 12 so as to partially face the differential lines 13a. 
In addition, the wiring conductor 13 formed on the outer layer side insulating layer 12 provided above the insulating plate 11 partially functions as a semiconductor element connection pad 17 which is to be connected to a semiconductor element. The wiring conductor 13 formed on the outer layer side insulating layer 12 provided below the insulating plate 11 partially functions as an external connection pad 18 which is to be connected to an external circuit board. The solder resist layer 14 is formed on the surface of the outer layer side insulating layer 12. The upper solder resist layer 14 has an opening 14a to expose the semiconductor element connection pad 17, and the lower solder resist layer 14 has an opening 14b to expose the external connection pad 18. When an electrode of the semiconductor element is connected to the semiconductor element connection pad 17, and the external connection pad 18 is connected to a wiring conductor of the external electric circuit board, the semiconductor element is electrically connected to the external electric circuit board. Consequently, the semiconductor element is operated in such a manner that a signal is transmitted between the semiconductor element and the external electric circuit board through the wiring conductors 13 and the differential lines 13a. 
When the signal flows through the strip-shaped conductor, an electromagnetic wave propagates from the transmission unit to the reception unit of the signal. This electromagnetic wave is generated not only inside the strip-shaped conductor but also in its periphery. Therefore, when the strip-shaped conductors extending in parallel come closer to each other along with the high-density wiring, the electromagnetic wave generated in the periphery of one strip-shaped conductor at the time of signal transmission could interfere with the other closely formed strip-shaped conductor, and a noise could be generated. Especially, due to higher frequency, the noise generation becomes more conspicuous. Therefore, the problem is that the semiconductor element is erroneously operated because the noise is generated in the signal being transmitted.